Lysolecithin has been employed as a cell membrane permeabilizing agent to enable rapid and quantitative assessment of internal antigens by immunofluorescence flow cytometry with little or no nonspecific immunofluorescence. Using this technique, we have demonstrated the intracellular expression of several lymphoid and tumor-associated antigens on cells that lack detectable expression of these antigens on the cell surface. Studies are currently in progress to determine the molecular form of the intracellular antigens, the possible biochemical alterations in the antigen as expression changes from intracellular to cell surface, and the regulation of antigen expression by normal and malignant cells. The potential of this technique for monitoring uptake of drug and toxin monoclonal antibody immunconjugates by normal and malignant cells following therapy with these agents, and the value of this procedure in screening for intracellular antigens such as oncogene products is also under investigation.